As known in the art, multi-layered, co-fired, ceramic circuit boards are fabricated from a stack of layers of ceramic dielectric tape sold under the trademark, GREEN TAPE, manufactured by the E. I. Du Pont Company of Wilmington, Del. Each layer of the ceramic tape has a thickness of about five mils (0.005 inch). The surface of each layer may have metallic conductors printed thereon that can be electrically interconnected by means of small holes (knows as vias) in one or more of the layers. The holes are filled with a conductive material. Such a co-fired, ceramic circuit board is described in U.S. Pat. No. 5,041,695 issued on Aug. 20, 1991 to J. A. Olenick.
Two types of co-fired, ceramic circuit boards are available. These are: (i) high temperature fire (typically below 1300.degree. C.) and (ii) low temperature fire (typically below 1000.degree. C.). The high temperature, co-fire technology is used for alumina and aluminum nitride ceramics, whereas the low temperature, co-fire technology is used for glass ceramics (ceramic filler filled vitreous glasses or devitrifying glasses). The conductor metallurgy for high temperature, co-fired boards is W or Mo-Mn, whereas low temperature, co-fired circuit boards utilize Ag, Au, AgPd or Cu conductors.
Shrinkage in volume experienced by the ceramic bodies after firing is a encountered in the fabrication of multi-layered, co-fired, ceramic circuit boards. This shrinkage, which occurs in the x and y surface dimensions as well as in the z thickness dimension of the respective layers, results from the release, during firing, of the air distributed between the particles as well as the organic binder material in the green tape. Such shrinkage is quite high, typically 10-15% for low temperature, co-fired circuit boards. While one can attempt to compensate for such shrinkage in the x and y surface dimensions by oversizing the area of the green tape layers, it is quite difficult to control shrinkage consistently. For instance, in order that the variation in the x and y dimensions of the multi-layered circuit board, after firing, be maintained within 0.1%, a degree of control that may be as large as one to two parts in one hundred in the amount of shrinkage is required. Because of this, the yield on co-fired circuit boards is low. In addition, the low temperature, co-fired circuit boards suffer from poor thermal conductivity and low flexure strengths associated with the glass ceramic.
A single layer of ceramic fired on a metal base, such as porcelain enamel-on-steel, is known in the art. Because the temperature coefficient of expansion exhibited by the steel, which comprises such a base, is relatively large, the composition of the ceramic layer fired on the steel base also must exhibit a relatively large temperature coefficient of expansion, in order to closely match that of the base. High barium, high-expansion glass-ceramics have been developed which may be used to fabricate such porcelain enamel-on-steel boards. While other ceramic composition systems are known that exhibit a relatively large temperature coefficient of expansion, high-expansion in these systems is typically achieved by the addition of heavy metal oxides, e.g., lead, barium, or alkali, e.g., sodium and potassium. However, this results in higher dielectric constants, and higher dielectric losses, and may lead to poor chemical durability, which makes such high-expansion systems poor candidates for use in fabricating multi-layered, co-fired, ceramic circuit boards that may be employed in microelectronic packaging.